Cardiomyocyte loss following myocardial infarction is a major cause of chronic heart failure (CHF), and therapies that protect or regenerate cardiomyocytes are needed to address this unmet clinical need. Numerous pre-clinical and clinical studies have revealed the importance of ErbB receptor-mediated signaling in both cardioprotection and cardiac regeneration. Thus, engagement of ErbB receptors is an emerging and potentially translatable strategy to treat CHF. We have engineered novel, bivalent ErbB ligands based on the crystal structures of ErbB receptors. The bivalent ligand approach uses two receptor binding domains covalently linked in a spatially constrained manner, such that each domain should be capable of binding separate receptors in a dimerized conformation. By capitalizing on receptor-ligand affinity interactions, these bivalent ligands can bias ErbB recepto signaling to activate downstream pathways preferentially compared to native ErbB ligands, unlocking new therapeutic opportunities. As shown in this proposal, we have now demonstrated the promise of this approach by showing that a bivalent NRG-1 (NN) induces a greater degree of ErbB4 phosphorylation in the heart compared to native NRG-1, resulting in functional cardioprotective benefit in vitro and in vivo in a mouse model. Moreover, NN induces cytostatic or apoptotic phenotypes in cancer cells in which native NRG-1 has pro-neoplastic effects. Thus, NN may lead to greater benefits in CHF treatment than native NRG-1, and understanding the mechanisms of this approach could lead to novel approaches to CHF therapy. Our Specific Aims are: Aim 1: To test the hypothesis that cardiomyocyte NRG-1b signaling can be biased to differentially activate molecular intracellular pathways relevant to CHF. Aim 2: To test the hypothesis that protein engineering applied to ErbB receptor ligands can differentially regulate cardiomyocyte ErbB receptor signaling. Aim 3: To test the hypothesis that bivalent NRG-1b (NN) is a superior CHF therapeutic compared to native NRG-1b.